FIELD OF THE INVENTION
The invention relates to a method for limiting the frequency of a voltage controlled oscillator in a control circuit of a resonant converter switched-mode power supply, and to a control circuit for a resonant converter switched mode power supply.
In a resonant converter switched-mode power supply, an oscillating circuit, which is typically a series oscillating circuit, is supplied through a first switch with a direct voltage by which the oscillating circuit is excited. After one-half of an oscillating period, the first switch is opened, and the oscillating circuit is connected to ground through a second switch, and as a result the capacitor of the oscillating circuit discharges again, and the oscillation is thus continued. An oscillation can accordingly be excited by alternating opening and closing of the two switches.
The coil of the oscillating circuit is provided by the primary winding of a transformer, and the winding ratio of the transformer is selected in such a way that approximately only the stray inductance appears on the primary side. The resonant frequency of the oscillating circuit consequently is determined by the stray inductance of the transformer and the capacitance of the capacitor.
Through the use of the secondary winding of the transformer, the energy is transferred to a load through a rectifier circuit. The energy transfer is accordingly effected by periodic excitation of the oscillating circuit, and more power can be transferred as the circuit is excited more often.
A decision as to how often the excitation takes place, or in other words the frequency with which the first switch is closed, is determined by a voltage-controlled oscillator, that in turn is triggered by a control amplifier which compares the output voltage of the switched-mode power supply with a reference voltage. The opening of the first switch and the closing of the second switch is brought about by a zero crossover detector, which detects the crossover from positive to negative values of the current by the oscillating circuit.
In the switched-mode power supply described above, the output voltage can be regulated by varying the frequency of the voltage-controlled oscillator. However, if the frequency is raised, then excitation of the oscillating circuit may already occur before the previous complete oscillation cycle of the oscillating circuit has ended, so that energy is taken from the oscillating circuit instead of being supplied to it. That situation arises if the frequency of the voltage-controlled oscillator becomes higher than the resonant frequency of the oscillating circuit for regulating the resonant converter switched-mode power supply, which signifies a turning point. In order to avoid reaching that turning point, the voltage-controlled oscillator with which the oscillating circuit excitation is controlled must be limited in its upper limit frequency.
That has been done heretofore by limiting the output voltage of the control amplifier. Although that method has the advantage of being able to be achieved with a simple circuit, nevertheless the resonant circuit, the oscillator and the limiter circuit must be tuned very precisely to one another. Moreover, production tolerances, for instance in terms of the stray inductance of the transformer and other components, can cause malfunctions at high load. At partial load, instead of the stray inductance, a higher inductance appears in the known circuit. A rapid transition from full load to partial load can cause malfunctions even if the dimensioning is correct. Limiting the control amplifier output voltage is usually performed with diodes. However, diode threshold voltages are temperature-dependent, so that the limitation functions cleanly only within a limited temperature range.